The invention relates to a cathode ray tube comprising in an evacuated envelope means to generate at least two electron beams which are converged completely or substantially completely on a display screen and are deflected over the display screen to write a field. Each electron beam is focussed on the display screen to form a spot by at least one focusing lens.
Such cathode ray tubes are used as colour television display tubes, as colour DGD display tubes for displaying symbols and/or figures (DGD=Data Graphic Display), as tubes having a high display rate for displaying computer data, or as projecting television display tubes.
Such a cathode ray tube is disclosed in U.S. Pat. No. 3,906,279 which may be considered to be incorporated herein by reference. This Specification describes an electron gun system for generating three electron beams, which system comprises three electron guns situated with their axes parallel and in one plane. As a result of the eccentric arrangement of the last electrodes of the outermost electron guns a two-pole component is associated with the lens fields in the focusing lenses of the electron guns as a result of which the outermost electron beams are deflected towards the central electron beam so that the three electron beams converge on the display screen.
U.S. Pat. No. 4,291,251 which may be considered to be incorporated herein by reference, is a cathode ray tube having a similar electron gun system in which the outermost electron beams are not converged in the focusing lenses but in the triode part of the two outermost electron guns. The triode part of an electron gun is formed by the cathode, the control electrode (g-1) and the first anode (g-2).
U.S. Pat. No. 3,011,090 which may also be considered to be incorporated herein by reference, discloses a cathode ray tube having an electron gun system with electron guns the parallel axes of which are situated at the same distance from each other. The last cylindrical electrode of the electron gun system is common to the three electron beams and together with the electrically conductive wall coating on the inner wall of the neck of the cathode ray tube constitutes an electron lens converging all beams. The effective diameter of the convergence lens is between the diameter of the last cylindrical electrode and the inside diameter of the neck with the electrically conductive wall coating. This latter will be further explained hereinafter.
U.S. Pat. No. 3,748,514 which may be considered to be incorporated herein by reference discloses a cathode ray tube in which the electron gun system comprises a long helical electrode for accelerating a large number of electron beams in such manner that space charge repelling of the beams is compensated for. In the last part of the helical electrode, all electron beams are simultaneously converged on and focussed on and then deflected over the display screen. The convergence and focussing is magnetic and occurs by means of a focusing coil around a part of the helical electrode situated on the display screen side. A disadvantage of this tube is that all electron beams simultaneously are focussed and converged by the same lens. Focusing and convergence are hence coupled so that dynamic convergence becomes impossible.
The manner of converging as described in U.S. Pat. Nos. 3,906,279, 4,291,251 and 3,011,090 have for their result that the spherical aberration in the electron beams increases. The convergence according to U.S. Pat. No. 3,906,279 moreover takes place while being coupled with the focusing.